1. FIELD OF THE INVENTION
The present invention relates to a stop device to be incorporated into a lens unit for a camera, and more particularly to the stop device having a plurality of stop blades disposed about an optical axis with an adjacent pair partially overlapping each other in the direction of the optical axis, each of the stop blades being pivotable about a pivotal axis positioned an edge region thereof relative to the peripheral direction of the optical axis to move toward and away from the optical axis, the stop blades together with defining an aperture opening with inner edges thereof.
2. DESCRIPTION OF THE PRIOR ART
According to this type of conventional stop devices, when the stop device is set to its full aperture position, a first inner edge portion of respective stop blades partially defines the full aperture opening, the first inner edge portion being formed arcuate to constitute along with first inner edge portions of the other stop blades a circular aperture opening having a predetermined radius and having the optical axis as its center.
Further, with this type of stop device, as illustrated in FIGS. 7 and 8 showing the prior art, a second inner edge portion a2 disposed more distantly from the pivotal axis than the first inner edge portion a1 and defining an aperture opening `A` when the stop blades are set to an intermediate aperture position in the middle of their pivotal range is formed straight tangentially along the first inner edge portion a1.
However, with the above-described conventional stop device, although in the full aperture position the arcuate first inner edge portions of the stop blades are placed on the same circle thereby together with defining a perfectly circular aperture opening without any corners, in the intermediate aperture position, as shown in FIG. 8, the small aperture opening `A` constituted by the straight second inner edge portions a2 is defined to be polygonal. This causes inconvenience to be described next when the stop device is incorporated into a lens unit designed for obtaining a photography having a delicate out-of-focus background image such as portrait photographing.
That is to say, although a portrait photographing using the above-described type of lens is sometimes carried out in the full aperture condition, in many cases, an intermediate aperture condition closed by a few steps from the full aperture condition is preferred for this type of photographing. If the aperture opening is defined polygonal in this intermediate aperture condition, in the worst case the defocused image in the background is defined by the shape of the polygonal aperture opening, conspicuously, or the out-of-focus background image is apt to be somewhat angular as being adversely affected by the polygonal aperture opening, whereby a delicate out-of-focus image desirable for the portrait photographing is not obtained.
The conventional stop devices have attempted to overcome this difficulty of the deterioration of out-of-focus image caused by the polygonal aperture opening by increasing the number of stop blades so as to define the aperture opening to be a nearly circular polygon. However, with this arrangement, because of the increased number of stop blades an adjacent pair of which partially overlaps each other in the closed aperture position, there often occurs damage or abrasion to the stop blades due to their increased mutual sliding contact.